PHAs are biodegradable and biocompatible, and their use for various molded products such as fibers or films has been studied. A great demand for a fiber produced from PHAs as a raw material can be anticipated as: medical equipment such as surgical sutures; fishery equipment such as fishing lines and fishing nets; clothing materials such as fibers; construction materials such as nonwoven fabrics and ropes; packaging materials for food or the like; etc.
PHAs, such as poly(3-hydroxybutyric acid) (hereinafter, also referred to as “P(3HB)”), are synthesized as intracellular reserve substances in many microorganisms found in nature. Such P(3HB) obtained by P(3HB)-producing microorganisms has been expected as raw materials for biodegradable products.
However, P(3HB) biosynthesized from a wild-type P(3HB)-producing microorganism has a number average molecular weight (Mn) of about 300,000 (i.e., a weight average molecular weight (Mw) of about 600,000). Such low-molecular-weight P(3HB) is rigid and fragile, so the fiberization thereof has been difficult.
In contrast, the present inventors have biosynthesized ultrahigh-molecular-weight P(3HB) of Mn=1,500,000 (Mw=3,000,000) from a recombinant Escherichia coli and have succeeded in convenient production of a P(3HB) film having improved physical properties in a reproducible manner (see Patent Document 1).
Further, as a process for fiberization of P(3HB), P(3HB) is melt-extruded, quenched, and solidified to form an amorphous fiber and the amorphous fiber is then cold-drawn almost at its glass transition temperature to orient the molecular chain of the amorphous fiber and subjected to a heat treatment, thereby resulting in success in convenient production of a P(3HB) fiber in a well reproducible manner. Further, in such a process, the use of ultrahigh-molecular-weight P(3HB) has lead to success in production of a fiber having improved physical properties, or a high-strength fiber (see Patent Document 2). Further, the use of ultrahigh-molecular-weight P(3HB) for performing further drawing after cold-drawing has lead to success in production of high-strength fibers with a high degree of elasticity (see Patent Document 3).
However, in those processes, there is a problem in that the fibers could not be provided with sufficiently high strength with respect to low-molecular-weight P(3HB). Therefore, a single-stage drawing is insufficient for obtaining a sufficient strength, so two or more stages of drawing should be carried out. However, the low-molecular-weight P(3HB) biosynthesized by the wild-type P(3HB)-producing microorganism is rigid and fragile, so it cannot be subjected to such a processing. Therefore, a process for obtaining high-strength fibers has been demanded regardless of the molecular weights of PHAs, which vary depending on origins such as a wild-type PHAs-producing microorganism product, a genetically modified strain product, and a chemical product.
Further, any of those processes require two- or more-staged drawing for obtaining sufficient strength, so the versatility thereof has been insufficient because of a large number of steps involved. Therefore, a process for more convenient production of a high-strength fiber has been demanded.
In contrast, processes for improving the physical properties of P(3HB) fibers by copolymerization of P(3HB) have been well studied. The copolymers of PHAs have been known to show a variety of physical properties by changing the types and compositions of monomers. Among them, poly[(R)-3-hydroxybutyric acid-co-(R)-3-hydroxyvaleric acid] (hereinafter, also referred to as “[P(3HB-co-3HV)]”) is commercially available as Biopol (registered trademark from Monsanto Co., Ltd.), having a tensile strength of 183 MPa, an elongation to break of 7%, and a Young's modulus of 9.00 GPa (see Non Patent Document 1). A fiber with a tensile strength of 210 MPa and an elongation to break of 30%, and a Young's modulus of 1.80 GPa has been reported as a fiber obtained from P(3HB-co-8%-3HV) by a process for simultaneously carrying out drawing and heat treatment, through the use of a continuous drawing machine after melt-extraction (see Non Patent Document 2). However, for using a copolymer fiber as a practical material, the copolymer fiber has been demanded to be further strengthened.
Non-patent Document 1: T. Ohura, Y. Aoyagi, K. Takagi, Y. Yoshida, K. Kasuya, Y. Doi, Polym. Degrad. Stab., 63, 23-29 (1999)
Non-patent Document 2: T. Yamamoto, M. Kimizu, T. Kikutani, Y. Furuhashi, M. Cakmak, Int. Polym. Processing, XII, 29-37 (1997)
Patent Document 1: JP 10-176070 A
Patent Document 2: JP 2003-328230 A
Patent Document 3: JP 2003-328231 A